Acid-base titrations

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Created by
Beth

Cards (46)

  • Reducing ability of halide ions and hydrogen halides

    Increases down the group
  • None of the hydrogen halides can reduce concentrated phosphoric acid
  • Chlorine (Cl) and ozone (O)

    • Can be used in water treatment to kill microorganisms
  • Acid-base titration

    1. One solution is placed in a burette
    2. The other solution is placed in a conical flask
    3. An indicator is added to the solution in the conical flask
    4. The solution in the burette is added to the solution in the conical flask
    5. The indicator will show the end-point of the titration (when the indicator changes colour)
  • Features important for safety

    • Use of a pipette filler
    • Use of gloves (where appropriate)
  • Features important for accuracy

    • Rinse the apparatus with the appropriate solution
    • Add the solution dropwise near the end-point
    • Swirl the flask and wash down the sides of the flask with distilled deionised water
    • Read the burette at the bottom of the meniscus
  • Features important for reliability

    • Repeat the titration two or three times
    • Concordant readings should be obtained within 0.1 cm³ of each other
  • Apparatus used in a titration

    Burette, pipette with safety filler, volumetric flask, conical flasks
  • Preparing a burette for use
    1. Rinse the burette with deionised water
    2. Ensure the water flows through the jet
    3. Discard the water
    4. Rinse the burette with the solution you will be filling it with
    5. Ensure the solution flows through the jet
    6. Discard the solution
  • Using a burette

    1. Read the volume at the bottom of the meniscus
    2. Hold the tap of the burette with the left hand to allow the right hand to be used to swirl to mix the contents of the conical flask
  • Preparing a pipette for use in a titration

    1. Use a pipette filler
    2. Rinse the pipette using deionised water
    3. Discard the water
    4. Rinse the pipette with the solution you will be filling it with
    5. Discard this solution
  • Using a pipette

    1. Attach a pipette filler to the top of a pipette
    2. Place the pipette in the solution and apply suction to draw the solution up
    3. Draw the solution up above the line on the pipette
    4. Release the suction until the bottom of the meniscus sits on the line
    5. Release the solution in the pipette into a conical flask
  • Volumetric flasks

    Used when diluting one of the solutions before the titration is carried out, or when preparing a solution of a solid
  • Carrying out a dilution of a solution
    1. Pipette the original solution into a clean volumetric flask
    2. Add deionised water to the flask until the water is just below the line
    3. Using a disposable pipette, add deionised water very slowly until the bottom of the meniscus is on the line
    4. Stopper the flask and invert to mix thoroughly
  • Dilution factor

    The amount the original solution is diluted by, calculated by dividing the new total volume by the volume of original solution put into the mixture
  • Preparing a solution from a mass of solid/volume of a liquid
    1. Weigh out an accurate mass of solid in a weighing boat, or measure out an accurate volume of liquid, and dissolve in a suitable volume of deionised water in a beaker
    2. Stir with a glass rod and rinse the weighing boat/cylinder into the beaker with deionised water
    3. Dissolve the solid/liquid in a small volume of deionised water
    4. Hold the glass rod above the beaker and rinse it with deionised water before removing it
    5. Place a glass funnel into the top of a clean volumetric flask and pour the prepared solution down a glass rod to the funnel
    6. Rinse the glass rod with deionised water into the funnel
    7. Rinse the funnel with deionised water
    8. Remove the funnel and add deionised water to the volumetric flask until the water is just below the line
    9. Using a disposable pipette, add deionised water very slowly until the bottom of the meniscus is on the line
    10. Stopper the flask and invert to mix thoroughly
  • Carrying out a titration
    1. Record the initial volume and final volume readings from the burette
    2. Use the right hand to control the tap by turning it slowly
    3. Swirl the conical flask with the left hand to mix the reactants
    4. Place a white tile underneath the conical flask to view the indicator colour more clearly
  • Steps in carrying out a titration

    • Rough titration (overshoot, but no more than 1 cm³ greater than the accurate titrations)
    • Subsequent accurate titrations (concordant within 0.1 cm³ of each other)
  • Standard solutions

    Solutions for which the concentration is accurately known
  • A burette has a total graduated volume of 50.0 cm³
  • If the rough titration is close to or above 25.0 cm³, it is essential to refill the burette before starting the first accurate titration
  • Recording titration results

    In a table with columns for initial burette reading, final burette reading, and titre
  • Important points about recording titration results

    • Put units with the headings, not in the main body
    • Record all values to 2 decimal places
    • Rough titration titre should be greater than the accurate titration titres but not more than 1 cm³ greater
    • Concordant titration titre values should be within 0.1 cm³ of each other
    • State the average titre with units to a minimum of one decimal place
  • Calculating the average titre

    1. Ignore the rough titration and any result that is clearly not within 0.1 cm³ of the other accurate titration values
    2. Write the average titre below the table and include the units
  • Units

    • Volume in cm³ (1 cm³ = 1 ml)
    • Concentration in mol/dm³ (M) or g/dm³
  • Uncertainty in measurements

    • Percentage uncertainty can be calculated for different pieces of apparatus
    • For a titre, the uncertainty is twice the uncertainty of the individual burette readings
  • Percentage uncertainty

    Calculated as (uncertainty/measurement) x 100
  • Percentage uncertainty calculations

    • 25 cm³ measured with 0.5 cm uncertainty: 0.5/25 x 100 = 2%
    • 25 cm³ measured with 0.008 cm uncertainty using a class A pipette: 0.008/25 x 100 = 0.032%
    • 0.120 g measured with 0.001 g uncertainty: 0.001/0.120 x 100 = 0.833%
    • 50 cm³ measured with 0.05 cm uncertainty: 0.05/50 x 100 = 0.1%
    • Titre (like temperature change) depends on two values, so total uncertainty is 0.05 x 2 = 0.1 cm³, which gives a percentage uncertainty of 0.1/50 x 100 = 0.2%
  • When two measurements are used to calculate a value then twice the uncertainty is appropriate
  • Strong acid

    Fully dissociated in solution
  • Weak acid

    Partially dissociated in solution
  • Common strong acids

    • HCl
    • H2SO4
    • HNO3
  • Common weak acids

    • CH3COOH
    • Organic acids
  • Common strong bases

    • NaOH
    • KOH
  • Common weak bases

    • NH3
    • Na2CO3
  • Acid-base titration

    1. Pipette known concentration solution into conical flask
    2. Rinse burette and fill with solution
    3. Titrate until indicator just changes colour
    4. Repeat for accuracy and calculate average titre
  • Volumetric calculations

    Number of moles = solution volume (cm³) x concentration (mol/dm³) / 1000
  • Degree of hydration titration
    1. Dissolve hydrated salt (usually hydrated sodium carbonate) in water and make up volume to 250 cm³
    2. Pipette 25.0 cm³ of sodium carbonate solution into a conical flask
    3. Titrate with 0.1 M hydrochloric acid using methyl orange indicator
    4. Use average titre to calculate moles of hydrochloric acid
  • Na2CO3 + 2HCl

    2NaCl + CO2 + H2O
  • Phenolphthalein and methyl orange are the two main indicators used for acid-base titrations